Frequently Asked Questions
Everything you need to know about the Sidereal Period Calculator.
What is a sidereal period?
The time a planet takes to complete one orbit relative to the distant "fixed" stars.
What is a synodic period?
The time between successive similar configurations of a planet as seen from Earth (e.g. opposition to opposition).
How does this calculator work?
Choose whether the planet is inferior (inside Earth's orbit) or superior (outside), enter one period, and it solves for the other.
Is it free?
Completely free, no sign-up, downloads, or limits.
Why are synodic and sidereal periods different?
Because Earth is also moving around the Sun, so relative alignments take longer (or shorter) to repeat than a planet's true orbital period.
What is Mars's sidereal period?
About 687 Earth days, though its synodic period (how often it's closest to Earth) is about 780 days.
What's the difference between a sidereal day and a solar day?
A sidereal day (~23h56m) is how long Earth takes to rotate relative to the stars; a solar day (24h) accounts for Earth's orbital motion too.
What happens if synodic and sidereal periods are nearly equal?
The calculator flags this rare edge case instead of dividing by a near-zero number.
Is my data stored?
No; everything runs in your browser and isn't stored or sent anywhere.
Does it work on mobile?
Yes, fully responsive on all screen sizes.